The Turnbull government's proposed plebiscite on same-sex marriage has formally been killed off by the Senate, where it was defeated 29-33 in a late-night vote on Monday, amid a warning the decision would delay marriage equality "for years".

Months of speculation and political posturing culminated in Labor, the Greens, the Nick Xenophon Team and Derryn Hinch combining to defeat the proposal, which would have seen same-sex marriage decided by the Australian people in February.

The decision - an inevitability since Labor pledged to oppose the plebiscite four weeks ago - will force a new conversation about marriage equality that will divide the Coalition and threatens to destabilise the Turnbull government.

Conservatives will resist any attempt to shift away from the plebiscite policy, demanding no action on same-sex marriage until at least the next election. Most observers expect any change on that front would blow up Prime Minister Malcolm Turnbull's leadership.

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The Senate votes on the same-sex marriage plebiscite on Monday.Credit:Andrew Meares

In a final defence of the policy he once opposed, Attorney-General George Brandis warned that a plebiscite was the only foreseeable way to achieve same-sex marriage and that if defeated, "the cause of marriage equality will be delayed for years".

Senator Brandis, a supporter of marriage equality, said Labor's decision to block the plebiscite was "one of the more cynical exercises in politics that I have ever seen". He implored the Senate to "stop playing politics with gay people's lives" and "get out of the way".

But senators had already made up their minds, voting 29-33 against the plebiscite just after 9.30pm on Monday. Australian Marriage Equality chairman Alex Greenwichsaid the focus would now shift to netting a parliamentary vote before the next election.

"The past 12 months has been a very difficult time for the LGBTI community," he said. "We now need to make sure that we channel the frustration of the plebiscite into passionate advocacy to achieve this reform."

Mr Greenwich said one positive to emerge from the past year was that several Coalition MPs had put on record their support for marriage equality.

The plebiscite was first floated by former prime minister Tony Abbott following a marathon Coalition party room meeting in August 2015, when liberal and conservative forces in the government tussled over how to respond to an issue which has long commanded the support of the Australian people.

Back then, Malcolm Turnbull had opposed the plebiscite in favour of a free vote, but was obliged to continue the policy when he took over as Prime Minister just a month later. Introducing the plebiscite bill in September, he said marriage equality was "a big moral issue" best decided by a public vote.

Earlier in the day, senator Dean Smith - the Liberal Party's first openly gay parliamentarian - spelled out his own opposition to the plebiscite, arguing it would set a dangerous precedent of the Parliament outsourcing important decisions.

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"Plebiscites are not and should not be a feature of our democratic culture in Australia," he said. "How do we look our electors in the eye and ask them to place their trust in us on future issues?" Senator Smith abstained from Monday night's vote.

Key crossbench senator Jacqui Lambie argued in favour of the plebiscite, saying that although she opposed same-sex marriage on "sacred religious grounds" she would have respected the will of the Tasmanian people after a plebiscite.